The present invention relates to a pressure sensor in which a differential pressure sensor chip for measuring a differential pressure and a static pressure sensor chip for measuring a static pressure are stored in one package.
Conventionally, as a method of measuring the flow rate of a fluid, a method is available with which a pressure difference between the upstream and downstream of a restrictor is measured by utilizing the fact that the pressure difference between the upstream and downstream of the restrictor provided to a flow channel is proportional to the flow velocity, and is converted into a flow rate. With a liquid level indicator, the pressure difference between the upper and lower portions of a tank is measured, and a liquid level with a liquid density being taken into consideration is measured from the pressure difference. For such differential pressure measurement, a differential pressure sensor is usually used. The differential pressure sensor is a sensor that receives two different to-be-measured pressures with sensor chips simultaneously and detects a differential pressure between the two pressures.
Sometimes a static pressure, i.e., a gauge pressure with reference to an atmospheric pressure, or an absolute pressure with reference to a vacuum state is measured simultaneously with the differential pressure described above, and monitoring and control are performed simultaneously. As the differential pressure sensor is a measurement unit that measures a pressure difference between two points, it cannot measure the static pressure itself.
A pressure sensor in which a differential pressure measuring sensor chip and a static pressure measuring sensor chip are combined is proposed as shown in Japanese Patent Laid-Open No. 63-008524 (reference 1). The pressure sensor disclosed in reference 1 is constituted by a sensor housing incorporating a differential pressure measuring sensor chip and static pressure measuring sensor chip, and a main body which is sealed with a pair of barrier diaphragms and into which a sealed liquid has been injected. The sensor housing and main body are fabricated separately, and are bonded and fixed to each other.
In the conventional pressure sensor described above, the interior of the main body is partitioned into at least two inner chambers to correspond to the pair of barrier diaphragms, and the sealed liquid is injected into the respective inner chambers. A high to-be-measured pressure applied to one barrier diaphragm and a low to-be-measured pressure applied to the other barrier diaphragm are transmitted to the differential pressure measuring sensor chip through the corresponding sealed liquid. Either one of the high and low to-be-measured pressures is transmitted to the static pressure measuring sensor chip through the corresponding sealed liquid. In this manner, as the conventional pressure sensor has a complicated structure, its package becomes undesirably large.
As the sensor housing and main body are fabricated separately, the distances between the barrier diaphragms and two sensor chips increase, and the distance between the two sensor chips accordingly increases. Therefore, in the conventional pressure sensor, the amount of sealed liquid increases, and accordingly the range of pressure that can be measured becomes narrow, so that the temperature characteristics are degraded.
To solve these problems, a differential pressure sensor and static pressure sensor may be integrally formed on one chip. To fabricate such a pressure sensor, one semiconductor substrate is etched to form a differential pressure measuring diaphragm and static pressure measuring diaphragm at different regions. Subsequently, strain gauges (in the case of piezoelectric pressure sensors) or electrodes (in the case of electrostatic capacitive pressure sensors) are formed on the two diaphragms.
When the differential pressure sensor and static pressure sensor are integrally formed on one chip, the manufacture becomes difficult and the cost increases. More specifically, the differential pressure sensor and static pressure sensor have different measurement ranges. For this reason, the static pressure measuring diaphragm must have a larger thickness and a smaller diameter than those of the differential pressure measuring diaphragm, so that it can withstand a large pressure.
When, however, the thickness of the diaphragm of the differential pressure sensor and that of the static pressure sensor are to be made different on one semiconductor substrate, the manufacturing process will become complicated. When the diameter of the diaphragm decreases, the step of forming a strain gauge or electrode on the diaphragm becomes difficult. When the two diaphragms have the same thickness and the diameter of the static pressure measuring diaphragm is increased, the manufacturing process can be facilitated. In this case, the sensitivity of the differential pressure sensor may decrease unless the diameter of the differential pressure measuring diaphragm is increased as well. When the diameter of the differential pressure measuring diaphragm is increased accordingly, the chip size increases, and the cost increases.
It is an object of the present invention to provide a pressure sensor that can obtain good temperature characteristics and a wide measurement pressure range.
It is another object of the present invention to provide a pressure sensor that can be manufactured easily and allows a package to be downsized.